I don’t think simply forwarding the port actually works with free Plex anymore. I think if the server has a different public IP from the client it asks you to pay, even if you’re connecting to the server over LAN.

Edit: That doesn’t appear to be true. I’m not entirely sure how Plex is checking whether you’re trying to stream remotely. In my case at least it works if I connect to my server using the LAN IP, but not if I use DNS (even though it resolves to the same IP). Maybe I’m missing something to allow it to work using the hostname.